The subject matter disclosed herein relates generally to preventing a fault in a DC drive power module, and, more particularly, to preventing a fault in a power module by inserting a crossover delay in a transition from forward to reverse and reverse to forward.
On DC drives, it is not uncommon to have fault problems related to power module gating (switching) at the transition between forward to reverse and reverse to forward. Problems with power module switching between forward to reverse and reverse to forward can be an issue at any size, but it is more prevalent in high horsepower systems and can become even more of a issue when a DC drive system is retrofit. A typical DC drive retrofit is where the power components, including transformers, motors, and power modules, remain in place, and the regulator is replaced with a new or updated regulator that provides improved command and control of the power components. However, newer, more modern regulators typically provide faster switching of the power module gates. With this faster switching provided by newer regulators, one of the forward or reverse power modules does not always have sufficient time to recover (completely turn off) before the opposite power module (the other of forward or reverse) is commanded to turn on. Larger power modules also typically require more recovery time.
A power module fault of this type is generally caused by “shoot through” currents between the forward and reverse power modules at the transition from forward to reverse and at the transition from reverse to forward. The power module that is being transitioned from does not fully turn off before the other power module is turned on. This problem can occur in power module configurations that include a transition between one or more power modules.
Known configurations attempting to address the shoot through problem simply insert a fixed time delay at the power module transition. However, this configuration presents a problem of its own because gate pulses to the power module can shift in time due to the regulator firing angle changes, which are a part of normal closed loop regulation. This presents the problem of re-enabling gate pulses to a power module at the end of the time delay yet in the middle of a gate pulse pattern, which could result in damage to the power module.
It would, therefore, be desirable to add a crossover delay at the transition point that would block the forward and/or reverse power module gate pulses to provide sufficient time for one power module to turn off prior to the next power module being commanded to turn on.